The present invention generally relates to an actuator, and more particularly to an actuator having an output shaft which performs a reciprocating linear movement by means of a screw mechanism.
Conventionally, as a source of linear movement, an actuator is known in which a reciprocating linear movement is obtained by means of causing an output shaft formed with an external threading to engage with a rotational gear formed with an internal threading. Such an actuator mainly employs an electric motor for a power source; and a rotational motion of the motor is converted into a reciprocating linear motion.
FIG. 1 is a cross sectional view of an example of a conventional actuator. A pinion gear 2 is fixed on an rotational shaft la of a motor 1, and a gear 3a formed on an outer portion of an internal threading gear 3 is engaged with the pinion gear 2. There is formed an internal threading (inner screw) 3b on an inner side of the internal threading gear 3. An external threading (outer screw) 4a of a screw shaft (output shaft) 4 is engaged with the inner screw 3b.
When the motor starts to rotate, the pinion gear rotates and the internal threading gear 3 engaged with the pinion gear 2 starts to rotate. As the internal threading gear 3 rotates, the inner screw 3b rotates.
The screw shaft 4 engaged with the inner screw 3b is not allowed to rotate, as a pin 4b provided on one end of the screw shaft is fitted into a guide groove 5a formed on a casing 5. Accordingly, by a screw principle, when the inner screw rotates, the screw shaft performs a linear motion in directions indicated by arrows A.sub.1, A.sub.2.
Conventionally, in an actuator having the above mentioned mechanism, a movement of a screw shaft is limited by causing an end of the screw shaft to come in contact with an inner surface of a casing, or by making a pin, which is provided for preventing a rotation of the screw shaft, come in contact with an end of the guide groove. In such a structure in which a movement of a screw shaft is caused to stop, a large fastening torque is applied to an engaging portion of the screw shaft due to an inertia of the screw shaft and an inertia of the motor when the screw shaft is caused to stop. This fastening torque sometimes exceeds the maximum starting torque of the motor. When such a condition occurs, the motor is not able to restart in a reverse direction (it is required, for starting of the motor, to move the screw shaft in a reverse direction) as the shaft is fastened with a torque greater than the maximum starting torque generated by the motor.
That is, in a conventional actuator of such type, a torque greater than the maximum starting torque of a motor is applied to a screw due to an inertia of the motor when the shaft is forced to stop by means of a stopper. For example, the applied fastening torque (the torque applied in order to stop the motor) is four or five times as large as the maximum starting torque of the motor. The unfastening torque (the torque applied in order to restart the motor) is, depending upon the coefficient of friction, approximately 80% of the fastening torque. Accordingly, an unfastening torque three to four times larger than the maximum starting torque of the motor is required; thus the motor is unable to start by means of only its own starting torque in this condition (unfastening torque&gt;starting torque).
Therefore, when a structure is employed, in which a screw shaft is forced to stop, it is common to use a ball screw that has a coefficient of friction smaller than that in an ordinal screw. However there is a problem in that ball screws are expensive and thus a manufacturing cost of the actuator greatly increases.
Additionally, in an actuator having the above mentioned structure, since there is a possibility of burning out a motor coil due to an excessive current flowing in the motor when the screw shaft is forced to stop, a protection circuit is provided for limiting an excess current. However, there is a problem in that a motor is not able to restart in a reverse direction due to an effect of the protection circuit.
FIG. 2 is a circuit diagram of an example of a conventional protection circuit. In the figure, numeral 1 denotes a DC (direct current) motor. One terminal of the motor 1 is directly connected to a driving circuit 8 for the motor 1, while the other terminal is connected to the driving circuit via a PTC (Positive Temperature Coefficient) thermistor 7 which serves for preventing burn-out of a coil of the motor 1. Accordingly, a current flowing to the motor 1 is supplied from the driving circuit 8 via the PTC thermistor 7.
The PTC thermistor 7 generates a heat when a current flows therein. Due to this heat, temperature of the PTC thermistor 7 rises, resulting in an increase of the resistance thereof. Accordingly, when a current flows to the coil of the DC motor 1, resistance of the PTC thermistor 7 increases and the current is restricted. Therefore, no excessive current flows to the DC motor 1 and thus the DC motor 1 is protected from an excess current.
As mentioned above, the conventional protection circuit has a thermistor connected in parallel between the driving circuit 8 and the PTC thermistor 7. In this structure, when supplying a current to a motor, in a reverse direction, immediately after supplying a current in a normal direction, resistance of the thermistor 7 remains at a value corresponding to the current flowing in the normal direction.
Therefore, there is a problem in that the motor is unable to restart in a reverse direction when resistance of the PTC thermistor is larger than normal due to an excess current for the normal direction having been supplied in order to force stop the motor.